1. Field of the Invention
This invention relates to a camera for silver halide film, a still video camera or an ordinary video camera having a visual axis detecting device capable of detecting a view point (visual axis) being observed by an observer (photographer) on an observation plane on which the object image by a photo-taking system is appearing.
2. Related Background Art
There have heretofore been proposed various devices for detecting what position on an observation plane an observer is observing, i.e., for detecting the so-called visual axis. For example, in Japanese Patent Application Laid-Open No. 61-172552, there is described a device in which a parallel light beam from a light source is projected onto the front eye part of an observer's eyeball and the visual axis is found by the utilization of the imaged positions of the corneal reflected image by reflecting light from a cornea and a pupil. FIG. 6 of the accompanying drawings illustrates a visual axis detecting method, FIG. 6A being a schematic view of a visual axis detecting optical system, and FIG. 6B showing the output signal of a photoelectric element array 6. In FIG. 6, the reference numeral 5 designates a light source, such as a light emitting diode, which emits infrared light not sensed by the observer and which is disposed on the focal plane of a light projecting lens 3.
The infrared light emitted from the light source 5 is collimated by the light projecting lens 3, is reflected by a half mirror 2 and illuminates the cornea 21 of the eyeball. At this time, the reflected light source image (virtual image) d by part of the infrared light reflected by the surface of the cornea 21 is transmitted through the half mirror 2, is condensed by a light receiving lens 4 and is projected onto a position d' on the photoelectric element array 6.
Also, light beams from the end portions a and b of an iris 23 form the images of these end portions a and b at positions a' and b' on the photoelectric element array 6 through the light receiving lens 4. Where the rotation angle .theta. of the optical axis T of the eyeball relative to the optical axis of the light receiving lens 4 (optical axis S) is small, when the Z coordinates of the end portions a and b of the iris 23 are Za and Zb, respectively, the coordinates Zc of the central position c of the pupil 24 are expressed as EQU Zc.perspectiveto.(Za+Zb)/2.
Also, the Z coordinates of the reflected light source image d and the Z coordinates of the center of curvature O of the cornea 21 coincide with each other and therefore, when the Z coordinates of the created position d of the reflected light source image d are Zd and the distance between the center of curvature O of the cornea 21 and the center C of the pupil 24 is OC, the rotation angle .theta. of the optical axis T of the eyeball substantially satisfies the following relational expression: EQU OC*SIN.theta..perspectiveto.Zc-Zd. (1)
Therefore, in a calculation processing device 9, the positions of particular points (the reflected light source image d and the end portions a and b of the iris) projected onto the photoelectric element array 6 as shown in FIG. 6B are detected, whereby the rotation angle .theta. of the optical axis T of the eyeball can be found. At this time, expression (1) is rewritten into EQU .beta.*OC* SIN.theta..perspectiveto.(Za'+Zb')/2-Zd', (2)
where .beta. is a magnification determined by the position of the eyeball relative to the light receiving lens 4.
Further, when the rotation angle .theta. of the optical axis of the observer's eyeball is calculated, the deflection of the visual axis relative to the optical axis of the eyeball is corrected, whereby the observer's visual axis is found. Also, in FIG. 6, there is shown an example in which the observer's eyeball rotates in the Z-X plane (for example, a horizontal plane), but the visual axis is also detectable when the observer's eyeball rotates in the X-Y plane (for example, a vertical plane), and an example of such method is also described in the aforementioned Japanese Patent Application Laid-Open No. 61-172552.
FIG. 7 of the accompanying drawings is a schematic view of the essential portions of a single-lens reflex camera when a visual axis detecting device is disposed in the camera. The object light transmitted through a photo-taking lens 101 is reflected by a retractable quick return mirror 102 and is imaged near the focal plane of a focusing screen 104. Further, the object light diffused by the focusing screen 104 is directed to the photographer's eye point through a condenser lens 105, a pentagonal prism 106 and an eyepiece 1. The visual axis detecting optical system is comprised of an illuminating optical system comprising a light source 5, such as an infrared light emitting diode emitting infrared light, not sensed by the photographer (observer) and a light projecting lens 3, and a light receiving optical system comprising a photoelectric element array 6 and a light receiving lens 4, and is disposed above the eyepiece 1 serving also as a dichroic mirror. The infrared light emitted from the infrared light emitting diode 5 is reflected on the dichroic mirror surface la and illuminates the photographer's eyeball. Further, a part of the infrared light reflected by the eyeball is again reflected by the dichroic mirror surface la and is condensed on the photoelectric element array 6 through the light receiving lens 4. From the image information from the eyeball (FIG. 6B) obtained on the photoelectric element array 6, the direction of the photographer's visual axis is calculated in the calculation processing device 9.
In this manner in a single-lens reflex camera, it is possible to know which point on the focusing screen the photographer is observing, in the case, for example, of a camera having a focus detecting device capable of accomplishing the focus detection of a plurality of points in the finder image field, it is effective, when the photographer tries to select one of points capable of focus detection which coincides with the main object (the object the photographer is going to photograph) and effect automatic focus detection, for saving the trouble of selecting and inputting one of those points, regarding the point being observed by the photographer as the point for focus detection to be effected, and automatically select said point to thereby effect automatic focus detection.
Now, it is usual for the recent single-lens reflex cameras to be used with a zoom lens mounted thereon, and the photographer zooms the lens to the telephoto or the wide-angle side in conformity with an object to be photographed to thereby determine the composition. Also, the movement of the photographer's visual axis and its distribution depend on the object to be photographed, and when for example, landscape is to be photographed, the photographer's visual axis moves substantially uniformly in the photographing image field, and when a person or the like in the foreground are to be photographed, the photographer's visual axis substantially concentrates upon the person to be photographed. Therefore, correlations can be found between the movement of the photographer's visual axis and its distribution and the then focal length of the lens.
In an interchangeable lens system, it is conceivable to mount a special lens, such as a macroscopic lenses or a fish-eye lens, in addition to ordinary lenses. It should also be taken into account that various adapters are sometimes mounted between a photo-taking lens and a camera body.
On the other hand, the photographer usually manually zoom a zooms lens mounted on a camera to the telephoto side or the wide-angle side to thereby determine the composition so that the size of an object to be photographed in the photographing image field may be a desired size. Recently, there have also beer realized commercially available cameras having the function of detecting the distance of an object and sending a signal from the camera body to the lens side so that the size of the object relative to the photographing image field may be constant, thereby automatically changing the focal length of the lens, and such cameras save the photographer the trouble of operating the zoom lens.
However, in a camera of this kind, the focal length of the lens thereof is set on the basis of the distance of an object detected by the camera body and therefore, when the object to be detected is mistaken, or when a plurality of objects are to be photographed at one time, or when the size of the object extremely differs from the size supposed during designing, there is the possibility of the lens being set to a wrong focal length.